Analysis of nucleic acids often requires amplification of a sequence or set of sequences prior to the more detailed downstream analysis. This can be because the sequences under investigation are rare in a sample, or because the downstream analysis requires a large amount of starting material. Samples obtained from biological sources or the environment can, however, include factors that inhibit amplification. In addition, there is variability between samples. For example, a blood sample from one individual may include more inhibitory factors than a blood sample from another, such that the pre-amplification step is less effective in the first sample. These issues can make downstream analysis more difficult or lead to inconsistent results between samples.